Question

If photosynthesizing green algae are provided with CO2 synthesized with heavy oxygen $_{ }^{ 18 }{ O }$, later analysis will show that all but one of the following compounds produced by the algae contain the $_{ }^{ 18 }{ O }$ label. That one is
(a) PGA
(b) PGAL
(c) Glucose
(d) RuBP

Answer 1

Utilization of ${ CO }_{ 2 }$ takes place in the light-independent reaction of photosynthesis known as the Calvin cycle. In this cycle, ${ CO }_{ 2 }$ is reduced by an acceptor molecule known as RuBP by the enzyme RuBisCO.

Complete Answer:
${ CO }_{ 2 }$ synthesized using heavy $_{ }^{ 18 }{ O }$ will become tagged with the heavy oxygen and all the products that are formed by this tagged ${ CO }_{ 2 }$ will also become tagged. As ${ CO }_{ 2 }$ is utilized in the process of the Calvin cycle, all the products of the Calvin cycle will contain $_{ }^{ 18 }{ O }$.
${ CO }_{ 2 }$ is reduced by RuBP to form PGA which is further reduced to form glucose. PGA, glucose and RuBP all are involved in the Calvin cycle and thus will contain $_{ }^{ 18 }{ O }$.
PGAL is the only compound that is not related to the Calvin cycle hence it will not contain $_{ }^{ 18 }{ O }$. PGAL is formed in one of the steps of glycolysis.

Additional information: Let us look at the Calvin cycle in detail.
Carboxylation : ${ CO }_{ 2 }$ is fixed into a stable organic compound known as RuBP by RuBisCO which further breaks down to give 2 molecules of phosphoglyceric acid PGA.
Reduction : This involves a series of reactions with the utilization of 12 ATP and 12 NADPH and 6 ${ CO }_{ 2 }$ to produce one molecule of glucose.
Regeneration : The acceptor molecule RuBP is regenerated with the utilization of 1 ATP.
So, the correct option is ‘PGAL’.

Note:
- The Calvin cycle was worked out by Calvin and his colleagues using radioactive carbon.
- RubisCO is referred to as the enzyme RuBP carboxylase-oxygenase as it has both reducing and oxidizing properties.
- ${ C }_{ 4 }$ pathway takes place in some plants where the first ${ CO }_{ 2 }$ acceptor is oxaloacetic acid (OAA).